The invention disclosed and claimed herein pertains to a system for controllably venting cryogen of extremely low temperature from a cryostat, wherein the system includes a gauge for monitoring cryogen pressure. More particularly, the invention pertains to a system of such type which employs simple and inexpensive means to prevent the gauge from becoming unreadable due to the icing or frosting over thereof.
In a superconducting magnet of the type commonly used in magnetic resonance imaging or spectroscopy, magnetic coils are contained in a cryostat and immersed in a liquid cryogen, such as liquid helium. The liquid helium is at a temperature on the order of 4.degree. Kelvin, and the extremely cold environment provided thereby maintains the conductors of the magnetic coils in a superconducting state.
A "quench" occurs when a substantial amount of the liquid cryogen in the cryostat goes into a gaseous state. Quenches are generally unintentional, and result in a sudden rise in cryogen pressure. The pressure must be rapidly relieved, and a flow path must be provided for the gaseous cryogen, to safely direct it to an exhaust or ventilator system so that it can be removed from the vicinity of the cryostat. In a common arrangement, a main cryogen vent line is coupled between the cryostat and the exhaust system, and is sealed by a burst disk. In the event of a quench, the pressure in the vent line immediately exceeds a disk bursting level, such as 20 psi, whereupon the disk is rupured and the gaseous cryogen is enabled to flow through the main vent line to the exhaust system.
When servicing operations are performed, such as to add additional cryogen to the cryostat (referred to as "filling"), or to couple electric current to the magnetic coils (referred to as "ramping"), some heat will be introduced into the interior of the cryostat. The amount of heat will not be enough to cause a quench, but will generate a small amount of gaseous cryogen. This small amount of cryogen must likewise be provided with a flow path from the cryostat to the ventilator or exhaust system. In some arrangements, the flow path is provided by coupling both ends of a disk bypass vent line to the main vent line, one end being coupled on either side of the burst disk. The small amount of gaseous cryogen generated by the servicing activity is thus routed around the burst disk, which is intended to seal the main vent line except for when a quench occurs. Typically, a valve is placed in the bypass vent line, to be opened during servicing activities of the above type, and otherwise to be kept closed.
It will be readily apparent that if the pressure of the cryogen generated during servicing activity exceeds the disk bursting level, the disk will break, even though a quench has not occurred. This is very undesirable, since all the cryogen in the cryostat would thereby be vented and be lost. The bursting disk could also pose a safety hazard to a service operator or other personnel who happened to be in the area at the time. Accordingly, a pressure gauge is placed in the disk bypass line, and is continually monitored by an operator as he performs his servicing tasks. The gauge will show that the pressure in the vent line is rising toward the disk bursting level, so that corrective action can be taken before the bursting level is reached.
The disk bypass vent line, as well as the fitting used to couple the pressure gauge into the bypass line, are typically made of a metal such as brass. While the cryogen flowing through the bypass line is in a gaseous state, it is still extremely cold, such as on the order of 20.degree. Kelvin. Accordingly, the fitting and bypass line become extremely cold, and heat is rapidly conducted away from the pressure gauge. The loss of heat causes the pressure gauge to frost or ice over. In some instances, frost collecting on the pressure gauge causes the gauge to resemble a "snow ball." The gauge thereby becomes unreadable, and is therefore unuseable for providing a warning of hazardous build-up in vent-line pressure.
In the past, a heat gun has been employed to keep the face of the pressure gauge clear from frost. However, such solution requires a service operator to perform an additional activity, and heat from the heat gun can cause distortion of certain of the mechanical parts of the gauge.